If magnetic field is generated by means of a cylindrical coil (or solenoid) consiting of N closely spaced turns, have a length l, and carrying a current of magnitude I,
Magnetic Induction (磁感应 Magnetic Flux Density)
Represents the magntude of the internal field strength within a substrate that is subjected to an H field.
magnetic moments (磁矩)
One is related to electron orbital motion around the nucleus; be a moving charge, an electron may be considered to be a small current loop, generating a very small magnetic field, and having a magnetic moment along its axis of rotation.
Each electron may also be thought of as spinning around an axis; the other magnetic moment originates from this electron spin, which is directed along the spin axis. Spin magnetic moments may be only in an "up" direction or in an antiparallel "down" direction. Thus each electron in an atom may be thought of as being a small magnet having permanent orbital and spin magnetic moments.
Diamagnetism (逆磁性/抗磁性)
Paramagnetism (顺磁性)
Ferromagnetism (铁磁性)
Antiferromagnetism (反铁磁性)
Ferrimagnetism (铁氧体磁性)
How the cassette works
PLAY
an already magnetized tape is passed beneath the head, by which a voltage is induced in the coil.
RECORD
a coil of wire around the ring carries the current to produce a magnetic field proportional to the signal to be recorded.
Re-recording
The magnetic material tends to hold onto any magnetization it receives and must be actively driven back to zero to start over. The large bias signal (typically 40 to 150 kHz in frequency) keeps "stirring" the magnetization so that each signal to be recorded encounters the same magnetic starting conditions
Stereo Playing
In a normal cassette player, there are actually two of these small electromagnets that together are about as wide as one half of the tape's width. The two heads record the two channels of a stereo program. When you turn the tape over, you align the other half of the tape with the two electromagnets.
Material Response to a Magnetic Field
Domain（磁畴）
Domain is microscopic in size, and for a polycrystalline material, each grain may consist of more than a single domain.
Hysteresis（磁滞）
Magnetic flux density vs. the magnetic field strength for a ferromagnetic material that is subjected to forward and reverse saturations. The hysteresis loop is represented by the solid curve; the dashed curve indicates the initial magnetization. The remanence Br and the coercive force Hc are also shown.
The Influence of Temperature on Magnetic Behaviors
Curie Temperature Tc
the temperature where a material's permanent magnetism changes to induced magnetism.
Néel Temperature TN
the temperature antiferromagnetic materials become paramagnetic.
